Electrodes for stimulating body tissue by electrical stimulation are known in great variety. For the utility of an implantable stimulation or sensing electrode—especially one intended for long-term use in a tissue stimulator with a non-renewable energy source and that, therefore, must require minimal energy—a high electrode capacitance and correspondingly low electrical impedance is of great importance. Furthermore, without sufficiently low impedance, a large voltage may cause polarization of both the electrode and the tissue to which the electrode is attached forming possibly harmful byproducts, degrading the electrode and damaging the tissue.
Because the ability of an electrode to transfer current is proportional to the surface area of the electrode and because small electrodes are necessary to create a precise signal to stimulate a single nerve or small group of nerves, many in the art have attempted to improve the ability of an electrode to transfer charge by increasing the surface area of the electrode without increasing the size of the electrode.
Due to platinum's superior electrical characteristics, biocompatibility and stability, it has often been used as a material for electrodes in corrosive environments such as the human body. While platinum has many desirable qualities for use as an electrode for electrical stimulation of body tissue, because platinum has a smooth surface, its surface area is limited by the geometry of the electrode, and therefore, it is not efficient for transferring electrical charge. The present inventor refers to this platinum with a smooth surface as “shiny platinum”.
One approach to increase the surface area of a platinum electrode without increasing the electrode size—and therefore to improve the ability of the electrode to transfer charge—is to electroplate platinum rapidly such that the platinum molecules do not have time to arrange into a smooth, shiny surface. Instead, the rapid electroplating forms what is commonly known as platinum black. Platinum black has a porous and rough surface which is less dense and less reflective than shiny platinum. U.S. Pat. No. 4,240,878 to CARTER describes a method of plating platinum black on tantalum.
Precisely because platinum black is more porous and less dense than shiny platinum, platinum black has weak structural and physical strength and is therefore not suitable for applications where the electrode is subject to even minimal physical stresses. Platinum black also requires additives such as lead to promote rapid plating. Lead, however, is a neurotoxin and cannot be used in biological systems. Finally, due to platinum black's weak structure, the plating thickness is quite limited. Thick layers of platinum black simply fall apart.
For the foregoing reasons there is a need for an improved platinum electrode and process for electroplating the electrode such that the electrode has increased surface area for a given geometry and at the same time is structurally strong enough to be used in applications where the electrode is subject to physical stresses.